Two million customers at the taxpayer-backed bank, which includes NatWest, have the app on smartphones and tablet computers, and many took to Twitter to express frustration after they were unable to use it to check their money.

One Twitter user wrote: "RBS app decides to play up on the 1 day of the month I need to know whether it's payday or not." A further tweet said: "Pointless having the #rbs app on my phone because it hardly ever works."

<noframe>Twitter: christine lamb - Pointless having the <a href="http://search.twitter.com/search?q=rbs" target="_blank">#rbs</a> app on my phone because it hardly ever works ð&Yuml;¡</noframe>

The glitch will be embarrassing for RBS, which last week announced that it was cutting 1,400 jobs in its retail arm as it moved investment into "things that matter most to customers" including mobile and online services.

RBS has been encouraging customers to use the technology, sending out tweets such as one two days ago which read: "Is it easy to pay someone with your bank's mobile app? It's simple with RBS."

Commenting on the glitch, a spokesman from Which? said: “Consumers increasingly use mobile banking services to access their accounts and will want to be assured that their money is accessible and safe at all times. These frequent glitches continue to raise questions about how robust and reliable banks’ IT systems are.”

RBS, which is 81pc publicly owned, hopes to move back into the private sector. Its latest annual results saw record losses of £5.2 billion, driven by a £390 million settlement for rate-fixing, £1.1 billion provision for mis-selling, and the IT problems of June and July 2012.

The bank's press office tweeted: "We are sorry customers are experiencing problems with our mobile banking app. We are working to restore service."

RBS apologised over the glitch as it scrambled to restore the service. A spokesman later said the app was working again. It appeared to have been out for around two and a half hours.

<noframe>Twitter: RBS Help - Sorry if you had trouble getting into Mobile Banking. Our service is now running as normal, thanks for your patience</noframe>

Customers using the NatWest brand also appeared to be affected, according to Twitter comments, while some also appeared to be experiencing problems with the Ulster Bank app.